Experiment: 8. Determining the Solubility of Aspirin at Different Temperatures and Calculating the Heat of Solution. Theory

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1 Experiment: 8 Determining the olubility of Aspirin at Different Temperatures and Calculating the Heat of olution Theory One of the most common forms of a homogeneous mixture is a solution. The one component of a solution, which is usually present in the greatest proportion, is called the solvent. The other components, present on a smaller scale, called solutes, are considered to be dissolved in the solvent. There are a number of different kinds of solutions: gases in gases (example: air), liquid in liquids (example: gasoline), gases in liquids (example: carbonated soft drinks), solids in solids (example: alloys such as brass), and solids in liquids (example: salt water). This experiment will involve a solution formed with a solid solute (a chemical salt) and a liquid solvent (water). olubility of a solid in a liquid is dependent on temperature, thus, at a given temperature, only a certain maximum amount of solute will dissolve in a given amount of solvent. Beyond that amount of solute, no more will dissolve and excess solute will remain in the solid form, settling to the bottom of the solution container. This maximum amount of dissolved solute, expressed quantitatively, is given in units of grams of solute/ g of solvent. uch a solution is termed a saturated solution When saturated solutions of solid solutes are prepared at elevated temperatures and then permitted to cool, the excess solute usually separates from the solution by crystallizing. However, if a saturated solution is prepared at an elevated temperature and any excess, undissolved solute is removed, crystallization often does not take place when the solution is allowed to cool undisturbed. The solution can contain more of the solute than normally is held in equilibrium with the solid state. uch solutions are said to be supersaturated.

2 olubility: The maximum quantity of solute that can be dissolved in a given quantity of solvent at a given temperature to make a saturated solution. Factors that Affect olubility: The Nature of olute and olvent The rule like dissolves like where like refers to similarities in polarities of substances, has useful application for predicting solubilities. In general, polar and ionic solutes tend to be more soluble in polar solvents and non-polar solutes tend to be more soluble in non-polar solvents. General Rules: Polar and ionic solutes are soluble in polar solvents Non polar solutes are soluble in non-polar solvents The Effect of Temperature ince most solutes have a limited solubility in a given amount of solvent at a fixed temperature (i.e. are not miscible), the temperature of the solvent generally has a marked effect on the amount of solute that will dissolve. For most solids dissolved in liquids, the dissolving process is endothermic and an increase in temperature results in an increase in solubility. Vant' Hoff Equation can be used to calculate the heat of solution as follow H log.33rt C and to calculate the heat of solution at two different temperature log H.33R T T where is the solubility at different temperatures (T in kelvin), H is the average heat of solution over the temperature range used, and R = 8.34 J mol - K -.

3 Tools and Materials Thermometer, hot water bath, pipette, burette, Aspirin tablet, NaOH, distilled water and phenolphthalein indicator. Procedure ) Grind up two aspirin tablet to a fine powder using the pestle and mortar. ) Prepare saturated solution form aspirin in ml distilled water at room temperature (5 o C). 3) Filtrate the aspirin solution and transfer ml from filtrated solution to conical flask. 4) Add -3 drops of phenolphthalein indicator solution (wirl for at least 3 minutes). 5) Titrate carefully with.5m sodium hydroxide (wirl the flask continuously, The NaOH solution should be added very slowly, The end point is reached at the first instance of the pink colour persisting). 6) Record the volume of the sodium hydroxide used. Repeat the titration procedure twice. 7) Repeat this procedure at 4 o C and 6 o C (using water bath for -5 min.). 8) To calculate the purity of aspirin in tablet: I. Measure out accurately ml of ethanol and transfer to the conical flask. Note: Ethanol is used in this case as aspirin does not easily dissolve in water. II. Add 5mL of distilled water to the conical flask. Finish the above procedure.

4 Calculations ) Calculate the aspirin concentration from this relation: (Aspirin) M =.5 V (calculated from burette) (NaOH) ) Arrange results as in the following table: aturated solution Temperature Aspirin concentration ) Calculate the aspirin solubility by the following equation: = M. wt of aspirin (8) M aspirin 4) Arrange results as in the following table: aturated solution Temperature /T Log () ) Draw a straight line between log and /T and calculate the heat of solution from the slope.

5 Log H slope.33r 6) Calculate the heat of solution for any two temperatures by the following equation: 7) To determine the percentage purity of aspirin in each tablet you will need the following information: At room temperature mole of aspirin reacts with mole of sodium hydroxide. % Aspirin in tablet = log /T H.33R T T Mass of aspirin calculated by titration in 35mL Mass of weighed aspirin tablet

Uniform properties throughout! SOLUTE(S) - component(s) of a solution present in small amounts.

54 SOLUTIONS - a SOLUTION is a HOMOGENEOUS MIXTURE. Uniform properties throughout! - parts of a solution: SOLUTE(S) - component(s) of a solution present in small amounts. SOLVENT - the component of a solution